Sergei Vassilvitskii
Authored Publications
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The streaming model of computation is a popular approach for working with large-scale data. In this setting, there is a stream of items and the goal is to compute the desired quantities (usually data statistics) while making a single pass through the stream and using as little space as possible.
Motivated by the importance of data privacy, we develop differentially private streaming algorithms under the continual release setting, where the union of outputs of the algorithm at every timestamp must be differentially private. Specifically, we study the fundamental $\ell_p$ $(p\in [0,+\infty))$ frequency moment estimation problem under this setting, and give an $\varepsilon$-DP algorithm that achieves $(1+\eta)$-relative approximation $(\forall \eta\in(0,1))$ with $\mathrm{poly}\log(Tn)$ additive error and uses $\mathrm{poly}\log(Tn)\cdot \max(1, n^{1-2/p})$ space, where $T$ is the length of the stream and $n$ is the size of the universe of elements.
Our space is near optimal up to poly-logarithmic factors even in the non-private setting.
To obtain our results, we first reduce several primitives under the differentially private continual release model, such as counting distinct elements, heavy hitters and counting low frequency elements, to the simpler, counting/summing problems in the same setting.
Based on these primitives, we develop a differentially private continual release level set estimation approach to address the $\ell_p$ frequency moment estimation problem.
We also provide a simple extension of our results to the harder sliding window model, where the statistics must be maintained over the past $W$ data items.
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We study differentially private mechanisms for sharing training data in machine learning settings. Our goal is to enable learning of an accurate predictive model while protecting the privacy of each user’s label. Previous work established privacy guarantees that assumed the features are public and given exogenously, a setting known as label differential privacy. In some scenarios, this can be a strong assumption that removes the interplay between features and labels from the privacy analysis. We relax this approach and instead assume the features are drawn from a distribution that depends on the private labels. We first show that simply adding noise to the label, as in previous work, can lead to an arbitrarily weak privacy guarantee, and also present methods for estimating this privacy loss from data. We then present a new mechanism that replaces some training examples with synthetically generated data, and show that our mechanism has a much better privacy-utility tradeoff if the synthetic data is realistic, in a certain quantifiable sense. Finally, we empirically validate our theoretical analysis.
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Measuring Re-identification Risk
Travis Dick
Adel Javanmard
Josh Karlin
Gabriel Henrique Nunes
SIGMOD(2023)
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Compact user representations (such as embeddings) form the backbone of personalization services. In this work, we present a new theoretical framework to measure re-identification risk in such user representations. Our framework, based on hypothesis testing, formally bounds the probability that an attacker may be able to obtain the identity of a user from their representation. As an application, we show how our framework is general enough to model important real-world applications such as the Chrome's Topics API for interest-based advertising. We complement our theoretical bounds by showing provably good attack algorithms for re-identification that we use to estimate the re-identification risk in the Topics API. We believe this work provides a rigorous and interpretable notion of re-identification risk and a framework to measure it that can be used to inform real-world applications.
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Transformer-based language models are being pre-trained on ever growing datasets (hundreds of gigabytes) using ever growing amounts of parameters (millions to billions).
These large amounts of training data are typically scraped from the public web, and may contain (public) personally identifiable information such as names and phone numbers.
Moreover, recent findings also show that the capacity of these models allow them to memorize parts of the training data. One defense against such memorization that has not yet been fully explored in this context is differential privacy (DP).
We focus on T5, a popular encoder-decoder, and show that by using recent advances in JAX and XLA we can train models with DP that do not suffer a big drop in utility, nor in training speed, and can still be fine-tuned to high accuracies on downstream tasks such as GLUE. Moreover, we show that T5's span corruption pre-training task, unlike next token prediction, is a good defense against data memorization.
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Scalable Differentially Private Clustering via Hierarchically Separated Trees
Chris Schwiegelshohn
David Saulpic
2022 ACM SIGKDD International Conference on Knowledge Discovery and Data Mining(2022) (to appear)
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We study the private $k$-median and $k$-means clustering problem in $d$ dimensional Euclidean space.
By leveraging tree embeddings, we give an efficient and easy to implement algorithm, that is empirically competitive with state of the art non private methods.
We prove that our method computes a solution with cost at most $O(d^{3/2}\log n)\cdot OPT + O(k d^2 \log^2 n / \epsilon^2)$, where $\epsilon$ is the privacy guarantee. (The dimension term, $d$, can be replaced with $O(\log k)$ using standard dimension reduction techniques.) Although the worst-case guarantee is worse than that of state of the art private clustering methods, the algorithm we propose is practical, runs in near-linear, $\tilde{O}(nkd)$, time and scales to tens of millions of points. We also show that our method is amenable to parallelization in large-scale distributed computing environments. In particular we show that our private algorithms can be implemented in logarithmic number of MPC rounds in the sublinear memory regime.
Finally, we complement our theoretical analysis with an empirical evaluation demonstrating the algorithm's efficiency and accuracy in comparison to other privacy clustering baselines.
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We present new mechanisms for label differential privacy, a relaxation of differentially private
machine learning that only protects the privacy of the labels in the training set. Our mechanisms
cluster the examples in the training set using their (non-private) feature vectors, randomly
re-sample each label from examples in the same cluster, and output a training set with noisy labels as well as a modified version of the true loss function. We prove that when the clusters are both large and high-quality, the model that minimizes the modified loss on the noisy training set converges to small excess risk at a rate that is comparable to the rate for non-private learning. We describe both a centralized mechanism in which the entire training set is stored by a trusted curator, and a distributed mechanism where each user stores a single labeled example and replaces her label with the label of a randomly selected user from the same cluster. We also
describe a learning problem in which large clusters are necessary to achieve both strong privacy and either good precision or good recall. Our experiments show that randomizing the labels within each cluster significantly improves the privacy vs. accuracy trade-off compared to applying uniform randomized response to the labels, and also compared to learning a model via DP-SGD.
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Secretaries with Advice
Paul Duetting
Proceedings of the 22nd ACM Conference on Economics and Computation (EC'21)(2021), pp. 409-429
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The secretary problem is probably the purest model of decision making under uncertainty. In this
paper we ask which advice can we give the algorithm to improve its success probability?
We propose a general model that unifies a broad range of problems: from the classic secretary problem
with no advice, to the variant where the quality of a secretary is drawn from a known distribution and
the algorithm learns each candidate’s quality quantile on arrival, to more modern ML-based versions of
advice where a binary classifier gives us noisy advice about whether or not the current secretary is the
best on the market.
Our main technique is a factor revealing LP that captures all of the problems above. We use this LP
formulation to gain structural insight into the optimal policy and present two case studies: a re-derivation
of the classic known distributions result with tools from linear programming and a tight analysis of the
noisy binary advice model.
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We study the problem of differentially private optimization with linear constraints when the right-hand side of the constraints depends on private data. This type of problem appears in many applications, especially resource allocation. Previous research provided solutions that retained privacy but sometimes violated the constraints. In many settings, however, the constraints cannot be violated under any circumstances. To address this hard requirement, we present an algorithm that releases a nearly-optimal solution satisfying the constraints with probability 1. We also prove a lower bound demonstrating that the difference between the objective value of our algorithm’s solution and the optimal solution is tight up to logarithmic factors among all differentially private algorithms. We conclude with experiments demonstrating that our algorithm can achieve nearly optimal performance while preserving privacy.
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Fair Hierarchical Clustering
Benjamin Moseley
Marina Knittel
Yuyan Wang
Neurips 2020
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As machine learning has become more and more integrated into our businesses and lifestyles, researchers have begun to recognize the necessity of ensuring machine learning systems are fair. Recently, there has been an interest in defining a notion of fairness that mitigates over-representation in traditional clustering.
In this paper we extend this notion to hierarchical clustering, where the goal is to recursively partition the data to optimize a certain objective~\cite{dasgupta}. For various natural objectives, we obtain simple, efficient algorithms to find a provably good fair hierarchical clustering. Empirically, we show that our algorithms can find a fair hierarchical clustering, surprisingly, with only a negligible loss in the objective.
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Sliding Window Algorithms for k-Clustering Problems
Michele Borassi
Neurips 2020 (to appear)
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The sliding window model of computation captures scenarios in which data is arriving continuously, but only the latest $w$ elements should be used for analysis. The goal is to design algorithms that update the solution efficiently with each arrival rather than recomputing it from scratch. In this work, we focus on $k$-clustering problems such as $k$-means and $k$-median. In this setting, we give simple and practical algorithms that come with stronger performance guarantees than previously known results. Empirically, we show that our methods store only a small fraction of the data, are orders of magnitude faster, and find solutions with cost only slightly worse than those returned by algorithms that have access to the full dataset.
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